1. Field of the Invention
The present invention relates to bone cements and, more particularly, to multi-solution bone cements and methods for making the same.
2. Description of the Related Art
The clinical use of total joint replacements in the United States is expected to rise precipitously over the next twenty-five years, projected to the level of over 4 million primary total knee and hip replacement procedures performed annually by the year 2030. The number of revision surgeries for both total hips and total knees will likely double over this time period as well. Thus, the demand for high performance bone cement is rapidly growing.
One of the critical factors in the clinical success of total joint arthroplasty is stable fixation of the prosthesis; which, in a majority of cases, is accomplished through the application of PMMA-based bone cement. While bone cement has been used clinically since the early 1960's and there are many commercially available powder-liquid cement compositions, the material continues to be scrutinized for the role that it plays in aseptic loosening of total joint prostheses.
Multi-solution acrylic bone cements (typically referred to as a two-solution bone cement, but which could have more than two solutions) have surfaced as an alternative to powder-liquid cement, using the same chemical constituents as current commercial formulations. This cement consists of PMMA powder pre-dissolved in methyl methacrylate (MMA) monomer, to form two separate solutions; one containing the initiator, benzoyl peroxide (BPO) and the other containing the activator, N,N-dimethyl-p-toluidine (DMPT), which react to initiate polymerization of the MMA when the solutions are mixed. These solutions have an initial viscosity similar to that of powder-liquid cement in the dough stage, therefore they can be simultaneously mixed and delivered to the surgical site via a single, closed system. This not only simplifies the surgical procedure by eliminating the multi-stage process of cement mixing and delivery, but also reduces the extent to which the properties of the polymerized cement depend on variations in surgical technique. Two-solution bone cement compares favorably to commercial cements (Simplex P and Palacos R) both in its mechanical properties and biocompatibility.
While the two-solution bone cement concept is a promising alternative to powder-liquid cements, it has several drawbacks in its current form, primarily related to the increase in monomer concentration necessary to form viscous solutions of dissolved linear PMMA. Many important properties of the cement, including the polymerization exotherm, residual monomer concentration, volumetric shrinkage, and shrinkage-induced porosity, are directly proportional to the initial monomer concentration. These properties represent the key areas where two-solution cement currently does not perform as well as commercial powder-liquid cements. The reduction of monomer in two-solution bone cement is limited by the solution viscosity, which is controlled by both the concentration and molecular weight (MW) of the PMMA in solution. Increasing the P:M ratio, without decreasing the MW of the PMMA, increases solution viscosity, yielding cements, which are difficult to mix and deliver. Significantly decreasing the PMMA MW in order to increase the P:M ratio, however, leads to a marked decrease in the mechanical properties of the polymerized cement.